The invention relates to error control in digital communication of speech data as in radio telephones. Bit errors occur in received speech frames due to communication channel disturbances. Error correction decoding of received speech frames is used to remove the bit errors prior to speech decoding of the speech frame. A speech frame that still contains residual bit errors after error correction decoding is termed a bad frame. If the bad frame is passed through the speech decoder unacceptable speech quality may occur due to distortions caused by the residual bit errors in the speech frame. To mitigate this problem received bad speech frames are identified as such prior to speech decoding by a binary bad frame indicator wherein a value of 1 signifies a bad frame. The speech decoder does not decode a bad frame but instead outputs previously decoded speech data or other substitute which is known to provide a more pleasant audio output than a decoded bad frame. The rate at which bad frames are indicated (BFI=1) is termed the frame erasure rate (FER). The GSM standard specifies stringent performance requirements for FER. The GSM requirement for BFI performance states that the number of undetected bad frames in a specified interval of time must be no greater than a specified number. Simultaneously, the GSM requirement states that the FER must be less than a specified number. In order to achieve an optimum compromise between these two opposing GSM requirements a bad frame criterion is needed.
In a conventional system, a cyclic redundancy code (CRC) is typically used to determine a bad frame. However, under the GSM standard, only a 3 bit CRC is provided which is a very weak code for identifying bad frames. A BFI indication based soley on the CRC cannot meet the GSM requirements for BFI and FER performance. In fact, the CRC may indicate no frame errors even though nearly half the frame bits are in error. This failure event can occur with non-negligible probability so that the CRC criterion alone does not suffice as a satisfactory BFI indicator.